It is known to burn oxidizable components of an exhaust gas or air flow in a cylindrical housing having an inlet port and an outlet port. The inlet port in such devices leads into a distribution chamber which is connected through heat exchanger tubes to a ring chamber neighboring an end of the housing, whereby these heat exchanger pipes are angled radially outwardly at the inlet ends and extend over a substantial portion of the length of the housing. A burner is arranged concentrically in the ring chamber and a flue gas mixing pipe formed as a cylindrical chamber section is arranged coaxially to the burner and facing the burner. The flue gas mixing pipe is surrounded by a main combustion chamber which in turn is surrounded by a ring space in which the heat exchanger pipes are located. The ring space in turn is connected to the outlet port.
German Patent Publication (DE) 3,532,232 C2 discloses further details of the just described combustion device. It is desirable to heat the exhaust air or gas carrying the oxidizable components to a preheating temperature as close as possible to the combustion chamber temperature. Such preheating of the exhaust gas or air imposes substantial problem with the mounting of the heat exchanger pipes in the housing, especially under dynamic operating conditions that is when the exhaust gas is subject to temperature changes, to volume flow variations, and to variations in the quantity of noxious components in the exhaust gas or air. These problems are due to the fact that the mounting of the heat exchanger pipes must compensate for or permit the thermal expansions and contractions of the heat exchanger pipes. If it is necessary to operate the apparatus with a very small exhaust gas volume it happens that heat exchanger pipes carrying a laminar flow are located directly next to heat exchanger pipes carrying a turbulent flow. As a result, different heat transfers take place at the pipe walls leading necessarily to different pipe wall temperatures with the result that even heat exchanger pipes located directly next to each other or neighboring each other are exposed to different thermal expansions, especially longitudinal expansions or contractions.
In order to permit an expansion of the heat exchanger pipes, especially a temperature responsive expansion, by structurally simple means, to thus protect the heat exchanger pipes against damage, the colder ends that is the inlet ends of the heat exchanger pipes are bent radially outwardly in the above mentioned known apparatus. Additionally, the exhaust air which has been cleaned of its oxidizable components and which flows around the outside of the heat exchanger pipes is lead over the pipe bends and over the bent away pipe sections.
The arrangement of the heat exchanger pipes in the known apparatus mentioned above, so that these pipes extend with their bent ends radially outwardly, has yet another disadvantage when it is necessary to avoid relatively large housing diameters. Space conditions requiring small diameter housings for the known device result in a very crowded arrangement of the heat exchanger pipes within the available small diameter. As a result, the combustion chamber as far as it is located inside the heat exchanger, has a very small volume. In other words, the main portion of the combustion chamber that is a large proportion of the total combustion chamber volume is located outside of the section which surrounds the heat exchanger pipe. As a result, housing diameters that must be kept as small as possible, conventionally result in housings that necessarily have a substantially increased length. Still another disadvantage is seen in that the radially outwardly bent legs of the heat exchanger pipes must be relatively short to accommodate the permissible housing diameter limits. As a result, when these short bent legs of the heat exchanger pipes are welded into an outer wall of the housing, their mounting becomes too rigid and they cannot compensate any heat expansions worth mentioning. At best, the conventional mounting is capable of compensating heat expansion differences among the heat exchanger pipes themselves. However, compensations that are required for accommodating expansion differences between the complete heat exchanger pipe bundle on the one hand and the jacket that surrounds the pipe bundle are not possible in the conventional device. As a result, it is necessary to equip the conventional device with a main expansion compensator which adds substantially to the costs of the conventional device.